Copper has been the main material used for concentric stranded conductors (rope lay concentric conductors) for electrical transmission used in automobiles and the like. In recent years, automobiles and the like are required to be lightweight in view of considerations such as energy-saving and, environmental preservation. Therefore, weight reduction of the concentric stranded conductor for electrical transmission is desirable. As a method for reducing weight, it has been proposed to use aluminum, which has small specific gravity, in place of copper.
An example is a concentric stranded conductor for electrical transmission that is excellent in bending resistance and vibration resistance and is resistant to breakage by friction and wearing at the time of bending and vibration (for example, see JP-A-2003-303515 (“JP-A” means unexamined published Japanese patent application)).
FIG. 2(a) is a partially cut-away perspective view showing the concentric stranded conductor for electrical transmission described in JP-A-2003-303515. FIG. 2(b) is a schematic cross section of the concentric stranded conductor. The concentric stranded conductor (1) for electrical transmission described in JP-A-2003-303515 is a concentric strand formed by twisting a plurality of single wires (3), (7), or (13) into child strands (i.e. a wire structure consisting of bunched or concentric configurations) and then twisting a plurality of the child strands. The concentric stranded conductor comprises a child strand at the center (central core strand (5) (a “strand” consists of any number of wires twisted together in the same direction with wires having the same lay length being located randomly)), a first concentric strand layer (11) formed around the child strand by twisting first-layer strands (9) so that the twist direction of their child strands (i.e. the twist direction of the single wires forming each child strand) is the same as the twist direction of parent strand (herein, a “parent strand” or “rope strand” is a final bunched or concentric configuration constituted by child strands, and “twist direction of parent strand” means the twist direction of the child strands forming the parent strand), and at least one concentric strand layer (17) formed around the first concentric strand by twisting second-layer strands (15) so that parent twist directions of adjoining layers are in the opposite direction and so that the twist direction of the child strands of each layer is the same as the twist direction of the parent strands.
Automobiles equipped with large capacity batteries, such as electric cars and hybrid cars, have appeared in recent years. Aluminum concentric stranded wires are also used in such vehicles as conductors for transmission of electricity from the battery. Since the amount of electricity conducted is large in these automobiles, a concentric stranded wire having a larger diameter than conventional ones is used. However, this raises the concern that the larger diameter may make attachment of the concentric stranded wire to the body of the automobile difficult. In addition, wires are required to be installed in a limited space. A concentric stranded conductor with better flexibility is therefore desired.